In order to better understand the origin of multiple quantum transitions observed in superparamagnetic nanoparticles, electron magnetic resonance (EMR) studies have been performed on iron oxide nanoparticles assembled inside the anodic alumina membrane. The positions of both the main resonance and forbidden (double-quantum, 2Q) transitions observed at the half-field demonstrate the characteristic angular dependence with the line shifts proportional to 3cos2q-1, where q is the angle between the channel axis and external magnetic field B. This result can be attributed to the interparticle dipole-dipole interactions within elongated aggregates inside the channels. The angular dependence of the 2Q intensity is found to be proportional to sin2qcos2q, that is consistent with the predictions of quantum-mechanical calculations with the account for the mixing of states by non-secular inter-particle dipole-dipole interactions. Good agreement is demonstrated between different kinds of measurements (magnetization curves, line shifts and 2Q intensity), evidencing applicability of the quantum approach to the magnetization dynamics of superparamagnetic objects.
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